Thursday, January 19, 2012

Does Warming Over the North Pole Lead to Colder Temps in Ohio?

Technical weather discussions aren't exactly front page material. When I try to explain the weather to my wife, the conversation abruptly ends of changes subjects. I get it. Its geeky.

Yet sometimes, we need the complex nature of the atmosphere to be explained so that the variables in a winter like this one can be handled a bit easier with ultimately better short and long term forecasts.

So indulge the geeky, scientist in me for a bit...the late winter forecast lurks at the end! :)

Most of our weather occurs in the lower 10 miles of atmosphere. This layer is called the troposphere. However, the behavior of the atmospheric layer above it--called the Stratosphere--plays a big role in major cold air outbreaks like the one in the winter of 2009.

The profile of the atmosphere that you may remember from middle school science class

Notice how the temperatures in the Stratosphere rise whereas the temperatures in the troposphere drop as you increase in height. When the temperatures in the stratosphere rise suddenly over the high latitude regions close to the north pole, often time this rise can result in a sharp drop in surface temperatures which can propagate south across the continent and into Ohio. Remember some of the below zero periods in our past winters? These warming events often enhance the cold outbreaks. I'll have examples of this later.

This sudden rise in temperature is called a SUDDEN STRATOSPHERIC WARMING EVENT.

There are several key components that go into identifying SSW events. 1. A sharp temperature rise of at least 50-70 degrees in a short time interval. And 2) is an abrupt reversal of the jet stream from west to east away from the pole. Since 1958, 31 Sudden Stratospheric Events have occurred according to this paper published in 2010.

Why does the stratosphere warm? The physics get very complicated and so do the equations that describe it. Here is the technical description from a paper I just read:

"The SSW is caused by a rapid amplification of planetary waves propagating upward from the troposphere. Planetary waves deposit westward momentum and create a strong meridional circulation which produces a large warming in the polar stratosphere due to adiabatic heating (e.g., McIntyre 1982)"

To visualize these "planetary waves", imagine a stream with random ripples on the surface caused by leaves, rock, sticks or any random object that permiates the surface of the water.

Imagine the atmosphere is like that stream filled with ripples. The only difference--and its a big difference--is that the atmospheric ripples are created by the earth's rotation and changes in heat from the oceans and land. These waves propagate like ripples throughout the atmosphere.

Courtesy: The COMET Program

When they travel up into the stratosphere (100,000 feet above the ground), they dissipate and give off heat as the flow decreases. This causes the warming described above.

Notice the warming that has taken place since December 15th

Here is where we are now...notice the sharp warming in the high latitude regions over the last week or so. The temperature graph is in KELVIN shown in the purple line. Last year's levels are in blue; the 30 year average is in black.

Also the change in the jet stream has begun but its not sharp...yet. If the purple line drops below zero, that will indicate sharp shift in the jet stream which might be enough to start pushing temperatures south.

I looked at several warming events of different intensities (not necessarily major events) to see how the different stratospheric warming events effects temperatures for us in northern Ohio and surrounding areas. The first year, 1987, didn't result in any cooling; the 2003 warming event lead to a much colder February.

2009 was a MAJOR warming event. Look at the sharp temperature jump and the abrupt "180 turns" in the jet stream. Did it result in a major cold air outbreak across the eastern US? Not really...cold air developed over the extreme southeast US

Each SSW event is very different and can yield different temperatures across the US and Ohio. Often times, the cold air that develops over the North Pole sinks over Europe or Asia. While cold air outbreaks aren't a foregone conclusion over Canada and the Great Lakes when a stratospheric warming occurs, the chances increase by more than 50% during and after an event according to the scientific paper I sighted earlier.

Can we use the ARCTIC OSCILLATION (a measure of the arctic's stability) to determine cold air outbreaks during a SSW event?

I plotted the AO levels for 1987 noting the SSW time period to see whether the AO reacted to it.

How about 2003?

And finally 2009, one of the largest warming events in recent memory...

While Stratospheric warming events do lead to drops in the ARCTIC LEVELS, the are not always sharp and sustainable. SSWs can lead to major cold air outbreaks but not always as indicated by the 2009 February temps

What does this technical weather stuff mean for our long term trends in Ohio?

The warming over the North Pole continues but is forecasted to bounce around a bit.

Take a look at the projections through January 21st.. Notice the colder than normal air building in Canada trying to push south and the above normal temperatures staying out west.

While I'm not jumping on the ARCTIC COLD bandwagon just yet, the trend given even a slight warming event would be for more frequent periods of colder temperatures in northern Ohio and more frequent Alberta Clipper type snows and more lake effect in February!